There are a number of applications in the communications and data processing fields in which information stored on punched paper tapes must be read. Individual data characters, each represented by a unique code of a fixed number of bits, appear on these paper tapes as lateral rows of punched holes. The presence or absence of holes in the respective bit locations of a given row determines the logical state of those bits and the identity of the character represented. Successive characters form successive equally spaced rows along the tape's length. Tapes of this kind are conventionally read by incrementally advancing the tape so that successive character bit rows appear in proper relationship to a hole sensing mechanism.
One conventional technique for tape advancement employs a stepper motor connected to a sprocket drive wheel. The sprocket teeth engage a separate series of sprocket drive holes positioned along the length of the tape. By incrementally advancing the sprocket drive motor through a fixed angle, the tape is advanced to permit sequential data characters to be read.
In such a tape reader, the tape must be accurately positioned for proper decoding. One conventional solution includes a drive motor mounted on a separate plate which, through a series of arcuate slots therein, may be angularly adjusted to effect the requisite alignment. Specifically, the motor mounting plate is attached to the main reader mechanism by screws extending through the arcuate slots. Alignment is accomplished by loosening the screws, rotating the motor until proper decoding occurs, and then retightening the screws. While this procedure has a superficial aura of simplicity, problems occur during manufacture rendering this technique tedious and expensive in production.
These problems arise as a consequence of the precision required for angular alignment. After the mounting screws have been loosened, the motor mounting assembly is relatively free to rotate about the drive sprocket axis. Precise angular adjustment can be rather difficult. Once the correct alignment position has been achieved, another difficulty is encountered in holding the assembly precisely as set while the retention screws are again secured. Any movement requires that the alignment procedure be repeated.
Finally, the very act of tightening the securing screws can, itself, create significant alignment shifts. Alignment errors introduced in tightening the retention screws are correctable only by a hit-or-miss procedure or by a lengthy iterative adjustment process.